Polymer is a npm package, and in a way, it’s a library that helps you create Web Components, but it’s even more complex than that. It is a whole system (which is maintained by Google) for you to work with Progressive Web Apps.

The Polymer project focus is in Progressive Web Apps and the maximum focus it’s in User Experience. The Polymer proposal it uses less Javascript and more the platform itself. The work is in HTTP 2, Service workers, mainly focusing in Offline First, also using others features of service workers. While you create your application, Polymer has components to create the documentation of it. Yes, you have support for that too.